In the manufacture of plywood, a thin layer of veneer is peeled in a continuous sheet from a log mounted on a lathe. The log blocks from which the veneer is cut are generally of a length equal to that of a desired finished plywood sheet. The continuous sheet of plywood veneer travels on conveyors adjacent a cutting knife or clipper which cuts the veneer transversely to the direction of travel of the sheet into incremental widths suitable for lay-up into a plywood sheet of desired width. Where the quality of the plywood veneer meets the specifications of the plywood component for which it is intended, the plywood clipper need be actuated only as necessary to achieve the desired width of the finished panel. However, most veneer as it is peeled contains defects resulting from knots, rot or splits in the log block. In order to eliminate voids and consequently reduced strength in the finished panel product, defects must be substantially removed.
In conventional practice, a clipper is actuated intermittently, in response to sensors which detect the defects. The clipper cuts out the defect portion by making incremental transverse cuts across the sheet, as often as necessary, to rid the veneer of the defects. The result is a collection of incremental pieces of veneer which must then be laid up edge-to-edge to form a panel component of the desired width. Three or more of the panel components are then typically sandwiched or layered together with an adhesive and pressed to form a finished panel having a requisite strength. In most mills, veneer strips that are at least 8 inches wide are usable in the lay-up process.
In a typical veneer production stream, up to 30% of the veneer will contain defects requiring removal. Conventional clipping apparatus and techniques reduce the loss of good veneer to about 10%. A significant portion of the veneer loss is due to inaccurate transverse cutting of the veneer. Also, present day apparatus lack the ability to cut out the narrower defects, which often are such that only a one-half inch width of veneer need be discarded.
Veneer mills and the plywood industry use two cutter or clipper concepts. The oldest concept employs a guillotine-type clipper blade which extends transversely to the direction of travel of the veneer and is actuated by an upstream sensor means which detects the defect. The guillotine impacts a stationary anvil surface which the veneer slides across as it travels under the blade.
The second conventional concept employs a rotating anvil roll and an upper, opposed roll upon which a knife blade is mounted extending transversely across the sheet. The veneer travels across the anvil roll, and upon a defect sensor actuating rotation of the upper roll, the knife blade rotates into contact with the anvil roll, thereby severing the veneer.
Hards, in U.S. Pat. No. 3,808,925, describes some disadvantages of the guillotine clipper and limitations of the rotating drum clippers. There are advantages to the guillotine-type clipper. For example, the rigid knife blade and frame of the guillotine clipper and its vertical action make optimum, accurate cuts through the veneer that are transversely aligned and perpendicular to the surface of the veneer. The rotating drum clippers, however, with a substantial radius of knife motion and adequate rigidity of knife mounting, only approach the quality of cut of the guillotine clipper.
The quality of the cut of the veneer is critical in subsequent processing or lay-up of the veneer into plywood. If the veneer strips are inaccurately cut, that is, not cut substantially exactly transverse to the continuous sheet of veneer, assembly of the resulting trapezoidal rather than rectangular veneers into a panel will leave gaps either at the ends of the panels or somewhere in the middle when the ends are squared up with respect to the edge of the outer surface sheets. Obtaining a cut that is perpendicular to the surface of the veneer, that is, a square-edged cut, is likewise important in plywood lay-up. A beveled edge has a tendency for overlapping adjacent veneer increments when they are crowded edge-to-edge to make up a finished sheet panel component.
The rigidity of the clipper blade and anvil assembly also affects the performance of the clipper over time. The more elastic the machine parts, the deeper the knife must be set to cut into the anvil in order to ensure complete transverse severence of the veneer. The transverse length of the cut, wood species, presence of knots, or cross grain, density and thickness affect cutting quality of the less rigid apparatus. Where a cut is unexpectedly easy, the knife cuts deeper into the anvil. The degree of such overcutting is in inverse proportion to the rigidity of the mechanism. Overcutting erodes the anvil material and must be compensated for by adjustments of the knife, and eventual replacement of the anvil. The less rigid apparatus typically have greater production downtime and maintenance costs.
As pointed out by Hards, alignment of the cuts transversely with respect to veneer flow is also dependent upon how well the drives on each end of the knife are synchronized and by the rigidity of the knife with respect to torsional deflection.
A remaining principal deficiency of all conventional clippers is the limitation of narrowness of the clips. Narrow defects must be removed from veneer with minimal waste of good veneer and reasonably narrow strips of acceptable veneer must be salvaged. The object of the Hards invention was to improve the narrowness of defect removal capability by reducing the radius of rotation of the knife to the least possible dimension.
Plywood mills often overspeed rotation of rotary clipper rolls to make narrower clips. The width of the defect that can be clipped by overspeeding the rotary knife is limited to about between only 2.5-4 inches at veneer flow rates commonly employed. Also, overspeeding of the rotary knife often causes disarrangement or misalignment of the veneer, especially as the narrower strips exit the clipper.
Recently, manufacturers of larger diameter and more rigid rotary clippers have included up to three knives on the knife holder drums in attempting to achieve narrower clipping capabilities. One manufacturer of veneer handling machinery, as shown by Colombo in U.S. Pat. No. 4,397,204, has combined two rotary clippers, each with three blades, and separate anvil rolls. While this system theoretically permits narrow defect clipping of about one inch or less in width, success is limited because the veneer loses register from one clip to the next. The veneer strips must transfer from a first anvil roll after the first clip to a section of conveyor and then to the second anvil for the adjacent, second clip. Precision of register or alignment for the second clip is difficult to maintain, and is a serious productivity limitation of the design. This machinery also suffers from complexity and high maintenance requirements.